How can the U.S. bishops say that we have no duty to attend Mass on a holy day of obligation if it falls on a Monday or a Saturday?
The 1983 Code of Canon Law for the Latin rite specifies 10 holy days of obligation: January 1, Epiphany, St. Joseph, Ascension, Corpus Christi, Sts. Peter and Paul, Assumption, All Saints, Immaculate Conception and Christmas.
That same Code allows episcopal conferences, with prior approval of the Holy See, to suppress certain holy days of obligation or transfer them to a Sunday (Canon #1246:2).
The precepts of the Church are interpreted by the Church's legitimate authority—in this case, by the Holy See and the bishops' conferences.